A calendar in a group is where members can originate and/or manipulate events, activity stream reporting of all calendar and event activity, geocoding (location services), several options (invitation, attendance information, location details, and reservation information can all be performed or utilized on a calendar invitation).
How is it different from other applications?
Like the other applications, it allows image upload. Calendar makes location geocoding available, unlike the other applications. It supports multiple event states like Ideation's ideas support multiple states. It has discussion, like forums. It provides a blank for an event's website. It does allow some space for event description, however it is not designed for long text pieces.
How do I create one?
Please refer to the procedure in the What is an application? article.